Electrochemical cell

ABSTRACT

An electrochemical cell is described, which provides a reliable gas generation even under unfavorable environmental conditions and with significant changes in temperature and environment. Furthermore the cell has a long life time. The electrolyte of this electrochemical cell comprises at least one ionic liquid containing thiocyanate ions for the generation of hydrogen cyanide gas. Preferred substances for the composition of the electrolyte and for the provision of thiocyanate ions in the electrolyte are specified.

FIELD OF THE INVENTION

The present invention relates to an electrochemical cell for generating hydrogen cyanide gas using a generator electrode and a counter electrode.

BACKGROUND OF THE INVENTION

Electrochemical cells for generating test gases for gas analysis devices are known, e.g. from German patent specification 26 21 677, for various gases, but not for hydrogen cyanide gas.

From document Z. Tocksteinová, F. Opekar, “The Electrochemical Generation of Small Amounts of Hydrogen Cyanide”, Talanta, 1986, vol. 33, no. 8, pages 688-690 a method for generating hydrogen cyanide gas using an electrochemical cell is known, wherein hydrogen cyanide gas is generated by oxidation of thiocyanate ions. The gas is generated via controlled oxidation of thiocyanate in aqueous solution according to reaction

SCN⁻+4 H₂O→6 e ⁻+SO₄ ²⁻+HCN+7 H⁺

A platinum electrode completely submerged in an aqueous electrolyte is flushed by nitrogen, so as to transport the generated hydrogen cyanide gas outward.

Besides the fact that the use of such a gas generator is only possible under laboratory conditions, this gas generator is unfavorable in particular due to the electrolyte which is an aqueous solution. Thus in practical applications for example climatic influences will cause loss of electrolyte, which does not allow for a reliable generation of hydrogen cyanide gas in controllable and constant amounts or might even cause a complete functional failure of such a generator.

SUMMARY OF THE INVENTION

Thus it is an object of the invention to provide an electrochemical cell which makes a reliable hydrogen cyanide gas generation with a long life possible even under unfavorable and substantially changing environmental conditions, such as significant changes in temperature and environment.

According to the invention the object is accomplished by an electrochemical cell of the type mentioned above, wherein the electrolyte comprises at least one ionic liquid and includes thiocyanate ions for generating hydrogen cyanide gas.

Due to its high electrochemical stability with respect to oxidation and reduction, the use of ionic liquids makes it possible to provide electrochemical cells for generating hydrogen cyanide gas, e.g. to check gas analysing devices, whereby the electrochemical cells generate stable amounts of hydrogen cyanide gas without failing over a long life time and also at changing temperatures and environmental conditions. The use of ionic liquid as electrolyte makes it possible to have long storage periods for such gas generators without impacting their function in later use.

According to a preferred embodiment of the invention, the electrolyte comprises a mixture of at least two ionic liquids, thus allowing to easily select and set the melting point and water absorption capacity of the electrolyte by an appropriate choice of ionic liquids and/or their appropriate mixing ratio. The electrolyte preferably comprises at least one ionic liquid of the group of 1-butyl-3-methylimidazoliurn trifluoromethanesulfonate (hereinafter abbreviated as BMIM OTf), 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (hereinafter abbreviated as EMIM OTf) and 1-ethyl-3-methylimidazolium thiocyanate (hereinafter abbreviated as EMIM SCN). These ionic liquids have been found to be particularly advantageous especially in view of the setting and adjusting options with respect to melting point and water absorption. In particular, the use of the mentioned ionic liquids and optionally of further additional substances results in reductions in the electroyte's melting point, which allows operation until far below the melting points of the individual ionic liquids and thus within the operating temperature range of the electrochemical cell, such as between −30° C. and +60° C.

Thiocyanate salt is preferably used as gas generating substance, and it is particularly advantageous to select it from the group NaSCN, KSCN, LiSCN, NH₄SCN, NBu₄SCN. However, thiocyanate can also be itself the anion of an ionic liquid.

It is particularly advantageous to have at least one noble metal as generator electrode, in particular one of the group of gold, rhodium, iridium, palladium or platinum, wherein platinum is particularly advantageous. The generator electrode is preferably formed as a platinum wire mesh or as a polytetrafluorethylene (in the following abbreviated as PTFE) gas diffusion membrane coated with the noble metal.

The container for the ionic liquid in the electrochemical cell is sealed to the outside by a PTFE gas diffusion membrane, which is permeable for the generated hydrogen cyanide gas. The generated hydrogen cyanide gas is discharged to the outside via this PTFE gas diffusion membrane.

It is advantageous to provide layers of glass fiber between the generator electrode and the counter electrode, the glass fiber being impregnated with the ionic liquid or with a mixture of ionic liquids, whereby these liquids comprise thiocyanate ions.

The counter electrode is preferably a PTFE support which is coated with platinum or platinum black, which is preferably porous.

DETAILED DESCRIPTION OF THE INVENTION

The present invention and preferred embodiments will be discussed below further with reference to the single drawing schematically showing an electrochemical cell according to the present invention.

An electrochemical cell 1 comprises a cell housing or container 2 at its bottom side above of which a counter electrode 3 is disposed on which ionic liquid 4 is provided. A generator electrode 5 is disposed on the side of the ionic liquid opposing the counter electrode 3. The generator electrode 5 acting as an anode and the counter electrode 3 acting as a cathode are connected to a constant current source 6.

On the outer side of generator electrode 5, which is formed for example as a wire mesh or as a gas diffusion membrane, another gas diffusion membrane is possibly provided, through which the generated hydrogen cyanide by diffusion passes out of the cell to the outside. The gas diffusion membrane 7 is additionally stabilized by a PTFE support grid 8.

The interior of the cell housing 2 containing the ionic liquid 4 preferably comprises glass fiber layers impregnated with ionic liquid containing thiocyanate ions.

The ionic liquid is selected in particular with regard of the melting point and the water absorbing capacity. BMIM OTf, EMIM OTf and EMIM SCN are of particular advantage. The ionic liquids can be used as a basis for the cell's electrolyte, either in its pure forms or as mixtures. It is preferred to have a proportion of 30-25 vol % of EMIM SCN in the mixture with EMIM OTf or BMIM OTf. Typically 0.2-0.4 mol/l lithium thiocyanate, potassium thiocyanate, sodium thiocyanate, ammonium thiocyanate or tetrabutylammonium thiocyanate are added to BMIM OTf. A proportion of 5-20% H₂O is required as a reactant in all cases. It is also used to optimize the solubility of thiocyanate salts and the reaction products.

Current densities of the current provided by the constant power source 6 and flowing through ionic electrolyte 4 are typically between 0.3 1.5 mA/cm³. The generated hydrogen cyanide gas passes through the PTFE gas diffusion membrane 7 by diffusion out of the cell to the outside. For a performance test of hydrogen cyanide detectors a generator electrode area of preferably 3.8 cm² and an operating current of about 2 mA with a power-on time of 30 s is advantageous, in order to generate hydrogen cyanide concentrations greater than 20 ppm in front of the outlet opening of the cell.

In the present embodiment the generator electrode consists of a platinum wire mesh or a coating of noble metal and preferably of platinum on the inner side of PTFE gas diffusion membrane 7. The counter electrode is a 0.25 mm PTFE gore membrane coated with platinum black. The electrochemical cell operates galvanostatically with current densities preferably between 0.5 and 1.5 mA/cm². With a generator electrode having a diameter of 20 mm the optimum operating current is 2 to 3 mA.

The invention has been described with reference to preferred embodiments. However, a person skilled in the art is capable of further embodiments or modifications without departing from the scope of the invention. 

1. An electrochemical cell (1) having an electrolyte (4), a generator electrode (5) and a counter electrode (3), wherein the electrolyte (4) comprises at least one ionic liquid and contains thiocyanate ions for generating hydrogen cyanide gas.
 2. An electrochemical cell (1) according to claim 1, wherein the electrolyte (4) comprises a mixture of at least two ionic fluids preferably from the group of 1-butyl-3-methylimidazolium trifluoromethanesulfonate, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate and 1-ethyl-3-methylimidazolium thiocyanate.
 3. An electrochemical cell (1) according to claim 1, wherein the gas generating substance is a thiocyanate salt.
 4. An electrochemical cell (1) according to claim 3, wherein the thiocyanate salt is at least one of the group of NaSCN, KSCN, LiSCN, NH₄SCN, NBu₄SCN.
 5. An electrochemical cell (1) according to claim 1, wherein the generator electrode (5) consists of a noble metal, preferably of one of the group of gold, rhodium, iridium, palladium and platinum.
 6. An electrochemical cell (1) according to claim 5, wherein the generator electrode (5) is a noble metal wire mesh and preferably a platinum wire mesh.
 7. An electrochemical cell (1) according to claim 5, wherein the generator electrode (5) is a PTFE gas diffusion membrane coated on one side with a noble metal.
 8. An electrochemical cell (1) according to claim 1, wherein it is sealed to the outside by a PTFE gas diffusion membrane (7), which is permeable to the generated hydrogen cyanide gas.
 9. An electrochemical cell (1) according to claim 1, wherein the ionic liquid (4) is provided in a housing or container (2) comprising glass fiber layers.
 10. An electrochemical cell (1) according to claim 1, wherein the counter electrode (3) essentially consists of platinum.
 11. An electrochemical cell (1) according to claim 10, wherein the counter electrode (2) is a PTFE coated with platinum. 